The possibility of increased packaging density and superior performance have created numerous applications for thin films on flexible substrates. In some of these applications, electrical circuits are formed on thin films which are deposited on flexible substrates while planar, and then folded or rolled into 3D package structures to increase microcircuit packing density. Flexible, in this context, refers to the ability of the substrate to be flexed (or bend) repeatedly without damaging the substrate. While the flexibility of the substrate material enables bending of the substrate, cracking of the thin film circuits due to flexure limits the durability of these flexible circuits. A crack in the thin film may increase electrical resistance of the thin film by decreasing the cross-sectional area available for the flow of current. The decrease in cross-sectional area available for current flow will depend upon the relative orientation of the crack with respect to the direction of current flow. If the crack is oriented parallel to the direction of current flow, the impact of the crack on current flow will be minimal. At other orientations, the crack will cause a larger increase in electrical resistance of the thin film.
The ductility of the thin film material deposited on the flexible substrate will dictate the amount of flexure a flexible circuit can tolerate before cracking occurs. While common conductors, such as copper and aluminum, have a high ductility, semiconductor materials and ceramic materials have a low ductility. The high ductility of common conductors enable thin films made of these materials to tolerate a moderate amount of flexure. However, the poor ductility (or high brittleness) of semiconductor and ceramic materials makes thin films of these materials (hereinafter referred to as ‘brittle films’) deposited on flexible substrates highly susceptible to cracking. In addition to increased susceptibility to crack initiation, the likelihood of crack propagation in brittle films is also higher.
One application of a brittle film on a flexible substrate is described in a co-pending application Ser. No. 11/490,135 ('135 application), filed on Jul. 21, 2006 titled “Thermoelectric Device” that is incorporated herein by reference. The '135 application discloses a thermoelectric thin film deposited on a flexible substrate that is wrapped around a support tube to create a thermoelectric element of a thermoelectric device. In such an application, the brittle nature of the thermoelectric material may induce cracks in the thermoelectric material during fabrication or winding. In a high volume environment, these and other similar films are typically deposited on a roll of flexible substrate within a deposition chamber. To enable deposition on a large area of the flexible substrate, the flexible substrate is bent (or otherwise flexed) within the deposition chamber. This bending of the flexible substrate with a deposited brittle film, cracks the brittle film. Even if a particular fabrication process minimizes flexure and associated cracking, handling of a flexible substrate with a deposited brittle film induces flexure and cracking of the film. These cracks typically propagate uncontrolled in random directions. Randomly oriented cracks may adversely impact the electrical characteristics of the film and decrease the durability of the film.
The present disclosure is directed at overcoming one or more of the shortcomings of the prior art thin films on flexible substrates.